Neutralization of deflection field between photocathode and mosaic of pickup tube



Aug. 8, 1950 G c szlKLAl 2,517,807

NEUTRALIZATION F EFLECTION FIELD BETWEEN PHoTocATl-IODE AND MOSAIC 0EPICKUP TUBES Filed May 30. 1945 difieren/v elf/v 6s @ANN/N6 i 74INVENTOR.

'fa/eos .Sz/KM/ Patented Aug. 8, 1950 NEUTRALIZATION '0F DEFLECTIONFIELD BETWEEN PHOTOCALTHODE AND MOSAIC F PICKUP TUBE George C. Sziklai,Princeton, N. J., assigner to Radio 'Corporation of America, acorporation of Delaware Application May 30, 1945, Serial No. 596,686

y iClaims. l.

The present invention relates to image-multiplier camera tube apparatusof the type used in television transmitting systems, and moreparticularly to a method and means for improving the resolution of suchtubes and hence the quality of the reproduced image.

Television pickup or camera tubes which operate upon theimage-multiplication principle generally include a photocathodeelectrode upon which an optical image to be transmitted is focused bymeans of a lens system. Light, falling upon the photocathode, causeselectrons to be released from the inner surface thereof in the form ofan electron image. The density of the electrons at any point in thiselectron image corresponds to the brilliance of the homologously locatedpoint on the optical image.

While an electron image is generally produced in image-multipliertelevision camera tubes in the above manner, it is converted into videoor image signal outputs in several ways depending upon the species ofcamera tube employed. In that type of camera tubeknown as the imageIconoscopef for example, the electrons released from the photocathodeare focused electrostatioally or electro-magnetically by means of anelectron lens upon a mosaic surface remotely spaced therefrom butusually located in a plane parallel to the photocathode. When theelectron image is caused to impinge upon the mosaic as a relatively highvelocity electron stream, it releases secondary electrons and soproduces on the mosaic electrostatic charges which are stored and whichare of magnitudes substantially proportional to the light intensity ofthe initial image at an homologously related point.

The mosaic is then scanned by a high velocity cathode ray beam developedwithin the camera tube. The scanning operation brings about the releaseof additional secondary electrons from the mosaic, with the number ofadditional 'seca ondary electrons released being dependent upon themagnitude of the electrostatic charge possessed by the particularelemental area of the mosaic being instantaneously scanned by thecathode ray beam.

The mosaic electrode of the image Iconoscope is provided With a metallicsignal coating or plate on the side thereof remote from that scanned bythe cathode ray beam. The changes in capacity between the elementalareas of the mosaic and this coating or plate due to the scanning actionabove explained provide a changing voltage at the signal plate so thatthe signal output Qi @he camera tube is obtained by connecting a loadcircuit thereto.

In another type of television camera tube known as the image Orthicon,the electrons released from the photocathode surface are drawnlongitudinally of the tube under the influence of an accelerating eld toimpinge on one side of a mosaic electrode. The impacting electronsstriking the mosaic at moderately high velocity cause secondaryelectrons to be released from the mosaic at the impacted areas, with therelease of electrons being proportional to the number of impactingelectrons. The released secondary electrons are collected upon a screenelectrode positioned adjacent the mosaic surface. The mosaic electrodeis of a somewhat different variety from that utilized in the previouslydescribed image Iconoscopa One preferred form of mosaic structure isdescribed in the copending application of Albert Rose, Serial No.407,132, led August 8, 1941, now U. S. Patent No. 2,403,239. As setforth in the Rose application. the mosaic is, broadly speaking, a glasssheet of minute thickness which receives on one side the impactingelectrons released from the photocathode. The impacting electrons causeelectrostatic charges representative of the optical image to betransmitted to be built up on the mosaic.

The charges are then released by the scanning action of a cathode rayscanning beam developed in the image Orthicon, which instead of scanningthat side of the mosaic electrode impinged by the electrons emitted fromthe photocathode, as in the image Iconoscope above referred to, scansthe reverse side of the mosaic. The scanning beam electrons, travelingat a relatively low velocity, impinge on this reverse side of the mosaicand restore each elemental mosaic area to an equilibrium state byneutralizing the negative charge deficiency existing thereon. The numberof scanning beam electrons collected at any instant depends on themagnitude of the negative charge deficiency possessed by the particularsurface area of the mosaic being instantaneously scanned by the cathoderay beam. Due to the differences in the amounts of positive chargeacquired by the various elemental areas of the mosaic surface, the totalnumber of electrons present in the scanning beam is not always requiredto neutralize a particular mosaic area. The excess beam electrons, or,in other words, those not collected by the mosaic, are returned towardthat portion of the camera tube in which the scanning beam is developed,where they are collected. The currents which are caused to flow in aconnected load circuit as a result of such collection of the electronstream constitute the signal output of the tube.

Each of the particular camera tube types above referred to accordinglyis so designed that electrons emitted from a photocathode electrode inresponse to the illumination thereof by a light image are caused toimpinge on a mosaic electrode which is spaced apart from thephotocathode. In passing between the photocathode and mosaic, theelectrons are in the form of an electron image representative of theoptical image. This electron image must be preserved substantiallywithout change in order to avoid distortion in the optical imagereproduced by the television receiver.

The scanning cathode ray beam, which is developed in each of theparticular types of camera tube above referred to, is periodicallydeflected horizontally at a relatively high rateV in order to scan themosaic electrode. Each scanning movement of the cathode rav beam occurs(for a 525 line image raster repeated 30 times per second) within a timeinterval of l/isflo of a second, and part of this time interval (usuallyless than is used to return the scanning beam to the starting point ofthe next scanning trace. While the line deflection (usually horizontal)of the cathode ray beam may be produced by electrostatic means, it isalso frequently accomplished electro-magnetically, in which case a splithorizontal deflection coil is placed across the neck of that part of thecamera tube in which the cathode ray scanning beam is developed. Asawtooth wave of current, iiowing through this deflection coil,`produces an electromagnetic iield which, by varying linearly with time,eiiects the desired linear line deiiection traces of the cathode raybeam relative to the impacted target.

Both of the camera tube types discussed above are customarilyproportioned so that the line or horizontal deflection coil is locatedin close proximity to the image section of the camera tube, or, in otherWords, to that section of the camera tube containing the photocathodeand mosaic electrodes. This design of the camera tube is made necessaryin part by limitations on the physical lengths of the paths traversed bythe electron image and the cathode ray scanning beam, as the difficultyof controlling an electron path usually increases with an increase inthe length of the path.

Due to the high rate at which the current in the line or horizontaldeflection coil changes in value, and due also to the proximity of thisdeflection coil to the image section of the camera tube, the magneticeld surrounding the line or horizontal deection coil has a detrimentaleiect on the accuracy with which the stored electrostatic charges on themosaic are-caused to represent an electron image correspondingpoint-forpoint to the optical image focused on the photocathode. Thisresults from the fact that the varying magnetic eld produced by thedeiiection coils gives rise to a corresponding varying lateraldisplacement of the electron image emitted from the photocathode priorto the impingement of this electron image on the mosaic. This high. rateof lateral oscillation, or jiggling, of the electron image during aframe sequence causes the reproduced optical image viewed at thereceiver to be blurred, since the electrons in the electron imagerepresenting a single elemental light area of the optical image notonlyv impinge on that one particular elemental mosaic surface areacorresponding to the single light area, but also on the elemental mosaicsurface areas contiguous to that one particular elemental mosaic area.This brings about a condition in which each elemental light area of theoptical image is, in effect, enlarged on the mosaic so that adjacentelemental areas overlap one another. Consequently, the resolution of thecamera tube is adversely ailected, and the quality of the reproducedoptical image at the receiver correspondingly lowered.

Attempts have been made to overcome the eiect of the scanning eld on theimage section of an image-multiplier television camera tube by shieldingthe deflection coil. While this expedient is satisfactory under certaincircumstances, it has been found that in many instances, especially inthe case of small image Orthicon tubes, no practical shielding willprovide adequate isolation of the electron image from the scanningfield.

According to a feature of the present invention, means are provided forovercoming the effects on the image section o an image-multipliertelevision camera tube of the magnetic iield produced by the line orhorizontal deiection coil. Furthermore, the means of the presentinvention eliminates the necessity for employing any shielding betweenthese parts of the camera tube assembly.

One object of the present invention, therefore, is to provide a methodand means for improving the resolution of a television camera tube ofthe image-multiplier type.

Another object of the present invention is to improve the accuracy withwhich the electrostatic charges on the mosaic electrode of animagemultiplier television camera tube are caused to represent anelectron image corresponding pointfor-point to the optical image focusedon the photocathode of the camera tube.

A still further object of the present invention is to provide a methodand means for overcoming the eiects on the image section of animagemultiplier television camera tube of the electromagnetic fieldproduced by the line or horizontal deflection coil associated therewith.

An additional object of the invention is to eliminate the necessity forshielding the line or horizontal deflection coil associated with animagemultiplier television camera tube from the image section of thetube.

Other objects and advantages will be apparent from the followingdescription of preferred forms of the invention and from the drawings,inwhich:

Fig.1 illustrates schematically one form of the present invention asapplied to an image-multiplier television camera tube of the typewherein a high Velocity scanning beam is utilized, such, for instance,as in the well known Iconoscope; and,

Fig. 2 illustrates schematically one form of the present invention asapplied to an imagemultiplier television camera tube of the type whereina low velocity scanning beam is utilized, such, for instance, as in thewell known Orthicon.

Referring rst to Fig. 1, there is shown an image-multiplier televisioncamera tube l0 of the high velocity scanning beam type, such as theIconoscope Many of the component parts of camera tube l0 for developinga video signal output are known in the art, and hence will not herein bedescribed in detail. However, tube I0 will be understood to include aphotocathode electrode I2 on which an optical image of an Object i4 iSfocused by means such as a lens I6. `Illumination falling onphotocathode I2 causes anemission of electrons from the inner surfacethereof, such emission, as is well known in the art, being in the formof an electron image each point of which substantially corresponds indensity to the intensity of the illumination on the corresponding pointof photocathode I2.

Electrons thus emitted from the inner surface of photocathode I 2 arefocused by means such asan electron lens I8 on one surface of a mosiacelectrode 20, as is known in the art. When the electrons` released fromphotocathode I2 impinge on mosiac 20, they release secondary electronsfrom the mosiac, and so form a charge deflciency on `the mosaic. Thesecondary electrons Ware collected'by a collector anode (not shown)which may be a metallic coating on a portion of the inner wall of tubeI0.

In another section of camera tube I 0, and spaced apart from thephotocathode I2 and lens I8, is an electron gun 22 w-hich may be of anysuitable type. Gun 22 develops a cathode ray scanning beam indicated inthe drawing by the reference character 24, The cathode ray beam 24 scansthe mosaic electrode 20, releasing from the latter additional secondaryelectrons. The variations in electrostatic charge on the mosaic 20 as aresult of the scanning operation above set forth are capacitivelytransferred to a signal plate or metallic coating 26 on the side ofmosaic 2D remote from that scanned by the cathode ray beam 24, andseparated therefrom by a suitable dielectric layer. The effects of thepotential changes on the signal plate 26 are then transferred over anoutput conductor 28 to an appropriate load circuit (not shown) toconstitute the video signal output of the camera tube l0.

The cathode ray scanning beam 2'4 is deflected at a relatively high ratein order to effect a lineby-line scanning of the mosaic 20. A similardeflection is brought about in a mutually perpendicular direction toproduce the field deflection so that the complete image raster istraced. This is not shown for reasons of simplicity and because it iswell understood in the art.

The means for producing this line or horizontal deflection includes adeflection coil 30 connected to a line or horizontal deflectiongenerator 32. Deflection coil 30 is generally of the split ortwo-section type, and is usually disposed relative to the neck of thatportion of the camera tube 50 containing the electron gun 22 so that thelongitudinal axis of the split coil is substantially perpendicular tothe normal :centered or undeflected position of the cathode ray beam 24.

The current output of generator 32 has a waveform of substantiallysawtoo-th configuration, as indicated by reference character 34. Thiscurrent, flowing through the deflection coil 30, produces anelectro-magnetic field, which, by varying substantially linearly withtime, effects the desired linear deflection of the cathode ray beam 24.

The cyclically varying magnetic field associated with the deflectioncoil 30, however, causes the electron image emitted from thephotocathode I2 to deviate from its normal path as explained above. Thiscondition is overcome in the present invention through the use of aneutralizing coil 36 connected to the deflection generator 32 in serieswith the cathode ray beam deflection coil 3|). The neutralizing coil 36preferably has rela- 4 tively few turns compared to the deflection coil30, and is arranged to produce an electro-magnetic field, which, in thevicinity of the image section of cameratube I 0, is in opposition to themagnetic field created by the line or horizontal cathode ray beamdeflection coil 30. By proper design and positioning of the neutralizingcoil 36 relative to the image section of camera tube I0, the auxiliaryelectro-magnetic field produced by the coil 36 may be made of equalstrength and in opposition to the line or horizontal deflection eld.Accordingly, the electro-magnetic elds of the two coils 30 and 36 willeffectively cancel one another in that section of camera tube I0traversed by the electron image emitted from photocathode I2. Thestrength of the electro-magnetic field produced by the neutralizing coil36, however, is insufficient to have'any appreciable effecten 4thedeflec-YW tion of the cathode ray beam 24 due to the action of the lineor horizontal deflection coil 30.

Fig. 2 shows one form of the present invention as applied to animage-multiplier television camera tube 5D of the low velocity scanningbeam type, such as that known in the art as the Orthiconf This type oftube, like the high beam velocity species referred to above, includes aphotocathode electrode 52 on which an optical image of an object 54 isfocused by means of a lens or lens system 56. Illumination falling onphotocathode 52 causes an emission of electrons from the inner surfacethereof in the form of an electron image similarly to the production ofthe electron image by the photocathode I2 of the camera tube IIJillustrated in Fig. 1.

The velocity of the electrons thus emitted from the surface ofphotocathode 52 is increased by an accelerating electrode 58 toward amosaic electrode 6U. The various electrodes of tubes I0 and 50 in Figs.1 and 2 respectively are maintained at their proper operating potentialsby any suitable t-ype of power supply, which has been omitted from thedrawing for the sake of clarity of illustration. A television cameratube of the low velocity type which is similar in many respects to thecamera tube 50 of Fig. 2, and including means for maintaining the properoperating potentials of the tube electrodes, is described in a copendingapplication of Robert R. Thalner, Serial No. 593,153, filed May 11,1945, now U. S. Patent No. 2,451,640, and the structure of the mosaicelectrode may be as disclosed in the abovementioned Rose application.

The electron image impacting mosaic 6U causes secondary electrons to bereleased therefrom. These secondary electrons are collected by a screenE2. The release of secondary electrons by a particular elemental area ofmosaic 5U leaves such area with a positive charge, the value of which isdependent upon the density of the electron image at that particularpoint.

The side of the positively charged mosaic 60 opposite to that impactedby the electrons released from photocathode 52 is then scanned by meansof a low velocity cathode ray scanning beam 64 produced by an electrongun B at the opposite end of tube 50 from the photocathode 52. Thestructure of the electron gun B6 may be of any suitable type known inthe art.

As the cathode ray beam 64 scans the side of mosaic 60 opposite to thatimpacted by the electrons released from photocathode 52, electrons fromthe scanning beam 64 neutralize the positively charged mosaic elements,the scanning beam supplying sufcient electrons to make up the negativecharge deficiency existing on each elemental mosaic area. If aparticular mosaic element is not positively charged, or, if suchpositive charge is small enough so that all lof the electrons availablein the scanning beam during the instant of passage are not required tomake up the negative charge deciency on that mosaic element, then theremaining electrons in the scanning beam, or, in other words, those notemployed to neutralize the electrostatic charge representing each imagepoint or element, are caused to return along a path substantiallyparallel to the scanning beam Sli toward the end of camera tube 50 fromwhich they were emitted. Upon arriving at the end of tube 50 containingthe electron gun 66, the returned electrons are collected by a signalplate 68 forming a part of the tube output circuit 10. v

The cathode ray scanning beam 64 of camera tube 50 is deiiected to scanmosaic $0 line-byline and also more slowly in a mutually perpendicularpath in substantially the same manner as is the scanning beam 2l! ofcamera tube IE! in Fig. 1. The line or horizontal deecting means in Fig.2 includes a line or horizontal deflection coil 12 mounted externally oftube 53 and located between mosaic Sil and the electron gun 65. The lineor horizontal deflection coil 12 may be split into two sections asillustrated, and is usually so positioned that the longitudinal axis ofthe coil is substantially perpendicular to the normal centered orundeiiected position of the cathode ray scanning beam 54, in a manneranalogous to the positioning of the coil in Fig. 1. A focusing coil (notshown) may be employed, if desired, to create an axial electro-magneticeld, so that the amplitude of deflection of the cathode ray beam 64 isproportional to the magnitude and axial length of the electro-magneticiield of the deflection coil 12, and inversely proportional to themagnitude of the axial electro-magnetic field produced by the focusingcoil.

Cyclically varying current for the line or horizontal deflection coil12, having a waveform of substantially sawtooth configuration such asindicated by the reference character 1t, is supplied by a line orhorizontal deflection generator 16. A neutralizing coil 18 is alsoconnected to the line or horizontal deflection generator 16 in serieswith the line or horizontal deection coil 12. The neutralizing coil 18is located exterior of the camera tube 5U, and adjacent to the imagesection thereof. The position of neutralizing coil 18 is so arrangedrelative to the envelope of camera tube 5l] that the electro-magneticeld produced by the coil 18 is substantially in opposition to theelectro-magnetic field produced by the cathode ray beam deflection coil12 in the vicinity of the image section of the tube 5U. Theelectro-magnetic iield of the neutralizing coil 18, however, does nothave any appreciable effect on the deflection of the cathode ray beam 64due to the action of the line or horizontal deflection coil 12.

In order to completely neutralize or cancel the oscillatory or jigglingeffect of the electromagnetic iield of the deflection coil 12 on theelectron image emitted from photocathode 52, the electro-magnetic fieldof the neutralizing coil 18 in the vicinity of the image section ofcamera tube 58 must not only be in opposition to the electro-magneticiield of the scanning coil 12, but it must be equal thereto in strength.In order to premit a variation in the strength of the electro-magneticeld produced by the neutralizing coil 18, so that the eld may be madeequal to the electro-magnetic iield established by deilection coil 12 inthe vicinity of the image section of tube 50, an adjustable resistor isprovided in parallel with the neutralizing coil 18.` By varying resistor80, the strength of the counter-deecting electro-magnetic eld producedby the neutralizing coil '18 may be made equal to the strength of theelectro-magnetic field produced by the deectio-n coil 12 insofar as itselect on the electron image emitted from photocathode 52 is concerned.Complete lateral stability of the electron image is thus brought about,and the resolution of camera tube 50 is materially improved as well asthe quality of the optical image reproduced at the television receiver.

It will be clear that, if desired, an adjustable resistor element, suchas indicated by the reference character 80 in Fig. 2, may be employed inparallel with the neutralizing coil 36 in Fig. 1. This permitsadjustment of the strength of the electro-magnetic eld produced by theneutralizing coil 36 in the same manner that the resistor 8U regulatesthe strength of the electro-magnetic iield of the neutralizing coil 18in the circuit of Fig. 2.

Having thus described my invention, I claim:

l. In a television system including a camera tube of the type having aphotocathode adapted to release electrons in response to the receptionthereby of light from an optical image, said electrons, in the form ofan electron ima/ge corresponding point-for-point in density tosaid-optical image, being then caused to impinge upon a mosaic electrodewithin said camera tube, said system also including means for developingand electro-magnetically deecting a cathode ray beam Within said tube toscan said mosaic electrode to thereby produce an output signal from saidtube representative of said optical image, the combination ofneutralizing means adapted to produce an electro-magnetic field, meansfor energizing said neutralizing means synchronously with said cathoderay beam deflecting means and means for positioning said neutralizingmeans relative to said camera tube so that the electromagnetic fieldproduced by said neutralizing means will tend to oppose theelectro-magnetic field of said cathode ray beam deflecting meanssubstantially only in that portion of said camera tube between andincluding said photocathode and said mosaic electrode.

2. A television system in accordance with claim 1, in which said cathoderay beam deiiecting means includes at least one coil positioned adjacentsaid camera tube and means for causing cyclically varying current to owthrough said coil to thereby produce a periodically varyingelectro-magnetic eld surrounding said coil. which acts to deiiect saidcathode ray beam, in which said neutralizing means also includes atleast one coil, and in which said positioning means includes means forpositioning ythe coil of said neutralizing means adjacent said cameratube and so arranged with respect to the coil of said cathode ray beamdeecting means that the varying electro-magnetic eld produced by thecoil of said neutralizing means in response to the synchronousenergization of both said coils will tend to oppose the varyingelectro-magnetic iield produced by the coil of said cathode ray beamdeflecting means substantially only in that portion of said camera tubebetween and including said photocathode and said mosaic electrode.

3. In a television system, a camera tube of the type in which electronsare released from a photocathode element to impinge on a mosaicelectrode which is then scanned by an electron beam developed Withinsaid tube, said electron beam being deflected to scan said mosaicelectrode by the action of an electro-magnetic field produced as aresult of cyclically varying currentv flowing through a iirst coilpositioned exterior of and adjacent to that portion'of said camera tubein which said electron beam is developed, a second coil positionedexterior of and adjacent to that portion of said camera tube traversedby the electrons released from said photocathode element in passing tosaid mosaic electrode. means for causing at least a portion of thecyclically varying current flowing through said iirst coil to also flowthrough said second coil. and means for so positioning the said coilsthatthe electro-magnetic elds of the coils in that portion of saidcamera tube traversed by the electrons released from said photocathodeare substantially in opposition.

4. In television apparatus, the method which comprises producing anelectronic current image representative of a subject, focusing theproduced electronic current image upon a target area there to develop acharge image under the control of the electronic current image,developing an electron beam and directing it toward the charge image onthe said target area, developing a iirst electromagnetic leld to deectthe electron beam according to a selected s-cansion pattern to scan thecharge image and thereby to release signals representative thereof to anoutput circuit, and developing a second electro-magnetic eldconcurrently with the rst electro-magnetic field and of equal strengthand opposite phase thereto in the region wherein the electronic currentimage ows so that the eifect of the said iirst electro-magnetic fieldupon the electronic current image is neutralized and nullified.

5. In television apparatus, the combination of means for producing anelectronic current 'image representative of a subject, means forfocussing the produced electronic current image upon a target area thereto develop a charge image under the control of the electronic currentimage, means for developing an electron beam and for directing it towardthe charge image on the said target area, means for developing a rstelectro-magnetic field to deflect the electron beam according to aselected scansion pattern to scan the charge image and thereby torelease signals representative thereof to an output circuit, and meansfor developing a second electro-magnetic leld concurrently with thefirst electro-magnetic eld and in the region wherein the electroniccurrent image flows so that the effect of the said first electromagneticfield upon the electronic current image in that region is neutralizedand nullied.

GEORGE C. SZIKLAI.

REFERENCES CITED The following references are of record in the ille ofthis patent:

UNITED STATES PATENTS Number Name Date 2,085,742 Farnsworth July 6, 19372,164,906 Deserno et al. July 4, 1939 2,220,303 Tingley Nov. 5, 19402,355,110 Rosenthal Aug. 8, 1944 2,387,608 Paumier Oct. 23, 1945 FOREIGNPATENTS Number Country Date 481,944 Great Britain Mar. 18, 1938 526,954Great Britain Sept. 30, 1940 806,582 France Dec. 19, 1936

